1. Field of the Invention
This invention relates to a mechanical pencil and the method of propelling a lead contained therein.
2. Description of the Prior Art
Mechanical pencils are known that have a barrel, a split tube within the barrel and a nut for propelling a piece of lead down the split tube. In those pencils where the lead may be propelled and retracted, a clutch is commonly provided at the forward end of the split tube to retract the piece of lead thereby also projecting the nut backwards, with rotation of the split tube. Rotation of the split tube in the opposite direction will project the lead. Typical of this type is the pencil disclosed in U.S. Pat. No. 2,511,301 issued June 13, 1950 to Smith. The barrel is provided with an internal screw-thread, and the nut is externally threaded to engage the internal screw thread within the barrel. Relative circular motion of the split tube causes the nut to move helically up or down within the barrel with the respective threads of the nut riding within the internal threads of the barrel.
Such a pencil has proved highly successful as a reuseable mechanical pencil, but the manufacturing costs of a plurality of parts including a clutch assembly, plus the assembly costs of units having a number of parts have caused these pencils to be too expensive for a disposable unit. As disposable ball point and fiber tip pens have proved a ready market for disposable writing instruments, there is a need for a disposable mechanical pencil which is competitive with the conventional wood pencils which do not provide the available lead advancing means of mechanical pencils and which obviates the necessity of repeated sharpening associated with the "wood case" pencil.